


At What Cost?

by maximum_overboner



Category: Undertale (Video Game)
Genre: Banter, Consentacles, Lighthearted, M/M, Wacky Shenanigans, gaster is mute, guilty sex, handjobs, one sided feelings, risky sex, sans is a lovable jackass, slight angst but not much, smutfic
Language: English
Status: Completed
Published: 2016-05-25
Updated: 2016-05-25
Packaged: 2018-06-10 14:18:06
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 6,179
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6960451
Author URL: https://archiveofourown.org/users/maximum_overboner/pseuds/maximum_overboner
Summary: <blockquote class="userstuff">
              <p>Workplace banter has a tendency to escalate to something more.</p>
            </blockquote>





	At What Cost?

**Author's Note:**

> A couple of anons requested Sanster where everything is fun and consensual, so here you go. It's completely separate from my main series, but I've thrown in a few neat references. It's also a little snoop into pre-core Gaster, but this is totally stand alone. Enjoy!

  The exterior of the Core was, much like the rest of the structure, hideously expensive. Sheets upon sheets of iron plating smothered the interior, a thick hide for the delicate machinery within, spread like woven silk from room to room, branching and clawing electronics in a place that should not have harboured them. The pipes were iron, the plating was iron, the interior was iron, if covered with tiles; all for the sole purpose of keeping the lava out and the scientists within from being toasted to a crunchy finish. It did that, and it did that well, the lava was kept at bay, but the issue was that the heat was trapped within the structure. And the closer you were to the base of it, the more unbearable the conditions became; labourers would quit after one week before droves rushed to fill the vacuum, eager for work in their limited world. The higher up the totem pole you were, the higher, and better ventilated, your workspace was. Dr. Gaster, as acting Royal Scientist, had the luxury of having air conditioning powerful enough to strip paint from the walls, and he enjoyed it. Sans, he could not help but notice, enjoyed it as well. Moreso. In fact, even though Sans had his own office as his Assistant, and was afforded most of the luxuries. Gaster peered out of his window over the crags in the rock, painted a soft burgundy by the magma.

  Sans was perched on Gaster’s desk, he noted with a tight lipped glance, craning towards the fan. A bead of sweat slipped from his forehead to mar the immaculately polished surface, his oversized labcoat scrunching underneath him. Sans let out a low groan, wiping at his forehead.

  Gaster turned, letting Sans see his gestures, his body an unearthly, foreign black.

  ‘Must you sweat all over my desk?’

  Sans chuckled. “it’s hot, so yeah.”

  ‘Now you know that’s not what I meant.’

  Sans grabbed the desk-fan, pulling it closer, dipping his head down to catch as much of the air as possible.  
  
  ‘Your office has air conditioning, doesn’t it? I made sure it did.’

  “i think it’s broken.”

  Gaster watched the lava bubble and pop, many feet below them, swirling and moving thickly like the tar that made up his body. He could spend hours there, just watching, overseeing, never growing tired of their patterns.

  “i mean, that, or the fact that i’m kinda done for the day. just wanted to hang out, you don’t seem busy.”

  Gaster quirked his brows, allowing himself a smile. ‘I’ll have you know I’m very busy.’

  “starin’ at the lava?”

  ‘Yes, it’s my new project.’

  “can’t help but feel you’re pullin’ my leg.”

  Gaster sighed, the smoke of his base moving, drifting to his chair before sitting down. He and Sans were now at eye level, with an ease borne of years of bantering and jokes that would appear vitriolic to outsiders.

  ‘God, you’re short. Remind me to divert some of the Core’s power, we may be able to build you a machine that would stretch you out to average height.’

  Sans chuckled. “rude. but yeah, i guess there’s nothin’ to do.”

  ‘You finished the paperwork I assigned?’

  “the necessary bits.”

  Gaster clasped his hands, leaning forward, bracing them to his face. He looked expectant. His silence always forced people to talk, even if they did not know it.

  Sans shrugged, reaching into his lab-coat, shifting so that he exposed the hollow cavity of his abdomen. Gaster tried not to look. With a sigh, Sans pulled out a balled-up wad of paper from the breast-pocket of his unironed shirt, before tossing it to Gaster with an underarm throw. Gaster snatched it out of the air with precision. Ew, it was damp.

  “nice catch.”

  Gaster unfurled the paper with one hand, not raising his gaze to meet Sans’ as he gave him a thumbs up in response. Sans watched as Gaster’s pupils slipped from one end of the paper to the other, his expressions always difficult to crack. They sat in silence, Sans swaying his legs from his point on the table and Gaster reading to himself, with no hems or haws to indicate any sort of overwhelming emotion. Finally, he sat the paper down, so he could sign properly.

  ‘The first ninety-five percent is, to be quite honest, astonishing. You have an observational knack the likes of which I’ve never seen. You explain incredibly complicated concepts concisely, if you remove every instance of the term ‘holy shit, check this out’, and that will always be remarkable to me.’

  “damn, i thought i already did that. and thanks doc, i appreciate it.”

  ‘The last five percent... Did you get bored?’

  “a little. how’d you tell?”

  ‘Because you trailed off in the middle of your sentence. Instead of elaborating on the magical properties of heat you drew a picture of yourself on a skateboard eating a hotdog.’

  “oh yeah, now i remember.”

  ‘It’s not half bad. I can’t even discipline you, I’m more impressed than anything.’

  Sans chuckled deeply, glad that his boss could take a joke. “heh, kinky.”

  Gaster went rigid in his chair before composing himself, a twitch so minuscule that the average person would not be able to pick up on it, a shudder in a body that was shifting and folding in on itself. It was an off-hand comment. Nothing more.

  ‘But I must insist that you get this done at some point.’

  “i will, i will. when have i not?”

  ‘You still have work outstanding.’

  Sans shrugged again, knowing full well that if he put the effort in he could blow through it. Gaster knew that too. For all their posturing about Sans’ laziness, it was difficult to ignore the consistent quality of his work, the innate talent of someone born to do something. He would be a good Royal Scientist, Gaster mused, decades down the line. Gaster couldn’t die through conventional means, but to pass the buck one day... Was only fair.

  A brief grimness overcame him as he thought of the Core, and he quelled it. He let his gestures loll, grinning, fine slips of black trickling through the cracks in his face.

  ‘Well, I suppose I must indulge you this once.’

  Sans made an appreciative noise from the base of his throat, his deep voice ringing out in the chamber of his jaw, and it made Gaster shudder.

  “ahh, thanks, doc. i like dozin’ in my own office and all, but sometimes you gotta get out and stretch your legs, y’know?”

  ‘I don’t have legs, I have a stalk to support my soul.’

  “sometimes you gotta get up and flex the ol’ soul stalk. wave it around a little.”

  Sans slipped off his coat, sighing, his shirt sticking to each bone individually, from his wide ribs to his slender collarbone, moving to his--

  Gaster looked out of the window once again. He let the seconds tick by in amicable silence, before slowly turning his gaze back to Sans. He looked at him as casually as he could.

  Sans stared back at him with the same narrowed gaze he would survey papers with, and Gaster felt his soul quicken, felt it thud and pulse in his chest. He would sweat, if he could.

  ‘Is something the matter?’

  “nah, nah. you ever... this is gonna sound strange, but you know how names sometimes just... fit people? like, you can look a dude on the street and think ‘he looks like an eric’, y’know?”

  ‘Yes?’

  “’gaster’ fits you.’

  ‘It is not my real name. It’s the one I ended up with, not that I mind.’

  “oh damn. you know your birth name?”

  ‘No . It’s... Probably something very mundane. ‘John’, or something.’

  Sans scrutinized him with his pinprick pupils before huffing. “nah, doesn’t suit you. ‘gaster’ does.”

  ‘I can assure you, I have no plans to change it.’

  Sans paused in thought.

  “’sans’ suits me, and ‘papyrus’ suits my brother...”

  Ah, Papyrus, dear, sweet Papyrus. Sans would gloat about him at every opportunity, about how proud he was, about how talented his cool brother was, about how he always, always tried his best, and though Gaster had never met him outside of a brief introduction, he could not help but like him. It was almost like hearing from a distant friend at this point, the stories were so numerous, but a light would fill Sans’ eyes as he would prattle off his tales that didn’t go anywhere, and so Gaster indulged him, just a tad. It was endearing. Sans had been present for groundbreaking, unheard of research into the nature of magic, and yet the look on his face did not compare to the times he had recounted Papyrus doing anything; playing sports (‘he kicked the ball into some dude’s face, but he was fifty feet away, so it was still amazing’), taking up knitting (‘accidentally got the needle wedged between his fingers, really did some squakin’...’) to his exercise regime (‘his discipline is insane, it is insane, dude, i dunno how he does it’). Sans doted on him. It was cute.

  Sans was cute.

  “... but if i had to change my name, i’d want it to be something subtle, dignified. uh...”

  Sans closed his eyes, holding his chin in earnest, serene thought.

  “’sandstorm fastfuck.”

  Gaster’s shoulder shook with laughter, his signing becoming sloppy.

  ‘Excuse me? You can’t call yourself ‘ _Sandstorm Fastfuck_.’

  “yeah, there’s a subtlety to it. i can say ‘sans’ is short version, and then whip that one out when people least expect it. you can be, uh...”

  ‘Oh God.’

  “’goopman pisslad’.”

  ‘I’m firing you.’  
  
  Sans snorted with laughter, giving Gaster a whack on the shoulder with his knuckle, fatigued from the heat and relaxing under the conversation. The warmth of his palm cascaded through Gaster until he was blushing like a schoolgirl.

  Lord, this was humiliating. Best to change to subject. He could put his reaction down to the ungodly humidity.

  ‘Papyrus, you said he was a... A soldier, was it?’

  “eh... a soldier in the makin’.”

  ‘Unemployed?’

  “unemployed. but he’s lookin’ to apply for the guard. he’s built like a brick shithouse. he's makin' friends with the captain.”

  ‘Out of all of the Human colloquialisms to infect our language, that is by far the worst.’

  Sans laughed, making a mental note to use it more in the future, to twist Gaster’s dials for his own amusement, borne of the harmless irritation of others, of pranks and japes.

  ‘I hope he does well. He would make a good soldier, if he is as determined as you say.’

  “he is. he sees a goal and just... goes for it, it’s amazin’. but i know he’s not gonna be frontline, he’s, ah...”

  Too naïve, from what Gaster had gleaned.

  “a real sweetheart, heh. you and him would probably get along great. he loves meetin’ new people.”

  ‘I would not mind meeting him. If he is like you, he would be excellent company.’

  Sans flopped his hand coyly, and Gaster knew he was playing up his reaction. But within the hyperbole, the exaggerations, the jokes and untruths there was always a kernel of sincerity. Digging it up was part of the fun, with Sans. Asking how he was was like unwrapping a present, it was never a clear-cut, and even the simplest niceties were wrapped in enigmas. He never talked about himself. Gaster himself had secrets, things he would never divulge, and he would never pry too deeply. Like attracts like.

  That is to say, like is attracted to like.

  Gaster sighed. He could not be dealing with crushes. He was hundreds of years old. Sans was twenty-five. He assumed that ‘crushes’ were something that fell away as you got older. And he would not indulge himself in such a manner, it was not done, and it was not right. He knew Sans’ entire life hinged upon this job, this opportunity, and for Gaster to do anything to compromise that would be nothing short of hideous. And he couldn’t, not whilst his soul beat in his chest. He couldn’t. He shouldn’t.

  He might?

  No.

  Sans picked between his teeth with his finger absentmindedly, a disgusting habit.

 _Cute_.

  Gaster wanted to punch himself in the face and then toss himself into the Core.

  Gaster reclined in his chair keeping his posture rigid, always rigid in a way that commanded respect from people that did not know him, the timeless, stalk of a Monster that gave off a sickly presence. He had considered himself exceptionally serious, but time had softened his edges somewhat, even if he could never shake of his reputation for being all-business, all the time.

  “yeah, well, if you’re gonna spank me for lazin’ about, get it over with,” Sans joked, head lolling under the sweltering heat.

  Oh, now that was cruel. It was too much, saying such things so casually.  
  
  Gaster set his hand on Sans’ shoulder, giving it a pat, a perfectly professional thud, two of the things, before motioning to draw his hand back but...

  Not. He let his hand lay there, and Sans looked at him blankly, processing what was happening, why he was feeling firm, uneven kneading at his shoulder; thin, solid fingers tapping to and fro. The touch drifted downward, towards his elbow, before travelling slowly towards Sans’ hand--

  Gaster felt his soul thud in his chest. He could see Sans’ ribcage shifting, his pupils wide.

  What was he _doing?_

  Gaster recoiled his hand, smoothing at his nonexistent clothing, attempting to wrench back his professionalism, cursing himself for that lapse in judgement, for his own stupidity, because that was a gross misuse of his status, of his position of Royal Scientist. Even if Sans were to reciprocate, what kind of boss--

  “uh... doc?”

  Gaster’s hand was frozen in the air, and he was furious at himself for his guard slip in front of his assistant, his scientific assistant, who despite being an adult was many years his junior.

  Sans was sweating, his breathing laboured, but his disposition cautious.

  “you... you okay, man?”

  ‘Sans, I am so, so sorry, that was incredibly inappropriate, I--’

  Sans grabbed one of his palms out of the air, holding it with his opposite hand awkwardly, as if they were shaking on a deal. They both looked at each other, Gaster leaning in his chair, Sans shifting on the desk, neither of them sure how to proceed.

  Slowly, looking at Gaster’s face for any sort of resistance, for any tenseness that would compromise the odd atmosphere that was building upon itself, the thick cloud of uncertainty. Gingerly, as if handling spun silk, Sans pulled Gaster’s palm towards his inner thigh, just above the knee, Gaster’s body being tugged to follow the motion

  Oh.

  Sans was warm. Shaking a little, too.

  Looking him in the eyes, in a state of total disbelief, Gaster trickled his hand upwards to follow Sans’ femur, before settling on the front of his pants. He felt something. A firmness.

  He pushed, just a tad, his arm limp and ready to be removed at the slightest hint of resistance, and Sans groaned. He inched his legs apart, reclining on his elbows, and his breaths were heaving and shallow.

  “s-so, i, uh... h-how far d’you wanna--”

  He yelped when Gaster kneaded at the tip of his cock over his pants, and Gaster could have listened to him do that forever. This was happening. He still couldn’t believe it, and he was certain that Sans was about to smack his hand away, like it were all a large prank at his expense.

  Sans pushed his hands away, and Gaster could smell musk, but his appreciation fell apart when the crushing shame cascaded in. Of course Sans didn’t want to, of course, of course, and it was selfish of Gaster to--

  Sans hopped off the table, tugging off the paperwork that stuck to him, before sweeping his entire arm over the surface, flitting all of the documents to the ground, letting them scatter and spread onto the floor. He turned to face Gaster, now ready. Gaster was, appropriately, aghast.

  ‘What was that for?’

  Sans quirked his brow. “so, y’know, we can fuck. it’s an old cliché, never heard of it? sweepin’ all the shit off the desk?”

  ‘Why couldn’t you just gather them up and put them on the chair? I... Think you broke my desk-fan.’

  “’cause this is faster. what, you want me to jizz on your lab papers? i could skip the middle man and just jerk-off onto ‘em, if you wanted--”

  Gaster was gathering the papers, form bent, and he would have grumbled if his hands weren’t occupied. He flicked through them, before neatly setting them down. He took a deep breath, before gliding forward to face Sans, entirely unsure of how to proceed, though his hesitance was falling away.

  ‘Look, if you feel like you have to do this for the sake of your job, you don’t--’

  He gasped, throwing his head back, latching his hands onto Sans’ shoulder as he pawed as his groin, fascinated.

  “what you got goin’ on, anyway?”

  Sans dropped down, squatting, running his finger slowly over the surface, the thick, inky fluid that collected on his palm dripping and coalescing back into Gaster.

  “that feel good, dude? i’m not sure what i’m doin’ here.”

  The long, shaky exhale he heard seemed a sufficient response as Gaster kneaded at his shoulders, overcome with lust and guilt. He could take a leaf out of Sans’ book for now.

  He would put off the guilt until later.

  Sans went on his toes, using his free hand to cup the back of Gaster’s head to draw him closer, and for one brief, wonderful second Gaster thought they might kiss. Instead, Sans leaned, dipping his face into the crook of Gaster’s shoulder to whisper something in that voice, that thick, deep voice.

  “whip out your spooky slime dick,” he breathed, “we’re doin’ this.”

  Gaster withdrew his hands from Sans’ shoulders so he could communicate, and his motions were fast and loose, slips and slurs that moved into each other. ‘I... Really, Sans?’

  “you gotta have somethin’ goin on. a silly-putty pussy?”

  Gaster looked at him flatly.

  “mystical magical asshole? i’m not picky, dude. whatever you’ve got, i can work with.”

  ‘Just... Just lie down, Sans.’

  “dirty talk not doin’ it for ya?”

  ‘Not from you, no.’

  Sans grinned wickedly, and though his face was fixed in a rictus the lust hit his eyes. His voice was deeper, gravelly, and Gaster couldn’t contain himself any longer, the heat, the need, was unbearable.

  “you aren’t gonna fire me if i’m a bad lay, right?”

  ‘You know, I hadn’t even considered the possibility.’

  Sans lay himself on the table, legs spread and dangling over the edge, propping himself up on his elbows. His erection tented the cloth of his pants. “of firin’ me?”

  Gaster leaned over him, his slick body looming in it’s foreignness. ‘No. Of you being a ‘bad lay’.’

  “ah, you’re flatterin’ me!”

  ‘A little.’  
  
  Gaster brought his hand to clasp roughly at Sans’ erection, his willpower finally snapping after three years of need, and the guilt settling in the pit of his body made the feeling that much sweeter, adding a forbidden tint to it. He clasped the whole thing and jerked roughly, the hole in his palm letting him see errant twitches. Sans gasped, his voice high, and his breaths were becoming shallow. He tugged again and again over the cloth, and it wasn’t true stimulation, but the novelty of the situation was enough to set them both off. Gaster felt his own cock twitch in time. But good things came to those that waited, and Gaster could wait with the best of them. He pulled his hand away yet again, his need to sign forcing him to do so, teasing them both. Sans’ eyes were half lidded, and a blush pasted his cheeks.

  ‘Your pants are an obstruction. Take them off.’

  “fine, doc.”

  ‘You can just say ‘Gaster’, you know, I believe we’ve been friends long enough for you to drop the formalities.’

  “aww!”

  ‘’Aww’ what, Sans.’

  “you called me your friend.”

  Gaster rolled his eyes, doing his best to conceal the tugging in his chest, the one he knew wasn’t reciprocated.

  ‘Considering what we’re about to do, I feel like ‘colleague’ doesn’t quite apply anymore.’

  Sans smiled as if he were helping someone across the street, as if he were doing Gaster a huge favour, and undid his belt. Slowly. Very slowly.

  He pulled on the leather strap, and Gaster watched the metal clasp give under the slight pressure, watch the belt buckle jingle. Gaster looked at Sans, cupping his face with such desperation that it almost looked aggressive. Sans knew what he was doing.

  Finally, finally, it was done.

  “guess i gotta undo my pants now, huh? you’re a real slave driver, gaster.”

  God, he had actually used his name.

  With a glare that could wither plants, Gaster undid Sans pants with a single hand, before pulling out his erection and jerking on it, roughly, firmly. Desperately.

  “oh-h, h-holy hell, you’re goin’ straight into it, huh? heh, some warnin’ would've been nice.”

  Gaster stopped, looking up with a dully amused expression. He mimed jerking off, pointed to himself, and then to Sans. ‘There you go,’ he signed, and resumed.

  “and you call me a smartass.”

  Gaster laughed, properly, from his belly, even if it was silent, and it made his motions uneven, shuddering. Sans could only look on, pleased with himself in that awful, smug way of his that made him so endearing. With a long, slow stroke, to punish Sans’ for his own sloth, Gaster jerked Sans off, drinking in the noises he made. He thumbed at his cock, slicking precum over the length to better aid his movements. Gaster watched Sans’ pupils roll into the back of his head with a low, long groan that broke into a loud moan, and he almost wanted someone to walk in because the day would be cemented as real in his mind. Gaster twisted his hand, running a long, thick tongue from the base to the top, to settle and lap gently at the tip, before moving to engulf--

  “g-gaster, doc, wait!”

  Gaster paused, still, ready to resume. Sans coughed. He had been fondling at his ribs, and his shirt was bunched up, sweat slicking every one of them. His mouth was open, just a tad, showing the tips of his teeth.

  “i-if you do that, uh... i won't be fit for anythin’ else, if you catch my drift.”

  Gaster cocked his brow, slipping Sans out of his mouth with a pop.

  “i-it’s, uh... i-it’s been a while, y’know?”

  He laughed, and it was shaky, and to his surprise Gaster picked up on his nerves. It could have been arousal.

  With a smirk Gaster stood, wiping at his mouth, thumbing at the corners, looking Sans up and down. He was a sweating mess, and the sickening heat was only somewhat responsible. Gaster sat beside him on the desk, pulling at his thigh to motion him to stand up, and with some signing and some prodding Sans was stood in front of him, Gaster’s thighs parted. Sans gulped, thumbing at his own dick as they spoke. He was staring at Gaster’s.

  “you got two hundred and six bones in your body... how ‘bout one more--”

  ‘No I don’t.’

  “i... what?”

  ‘I don’t have any bones in my body at all. Your pickup line, whilst appropriate, is moot. It isn’t even a real ‘bone’, you are talking about your penis.’

  Sans sighed, his gleaming white skull tinged orange under the light of the lava pools.

  “why you gotta rain on my parade, gaster?”

  Sans groped at Gaster, running his hand up and down, just to experience the novelty. Gaster had no form, and yet did; it was gelatinous, moving around certain fixed points, his face, his arms, his hands.

  “right, uh... so how is this workin’, exactly? ‘cause i have a hard-on, but i have no idea where to stick it.”

  ‘Romance is not your strong point, is it?’

  “god no.”  
  
  His hand settled at Gaster’s groin, in the space below his cock, and he found something he could work with.

  “i’m not usually one to do this kinda thing, y’know, all the work...”

  ‘All the work’, good God. Sans knew how to poke people’s buttons.

  “but, uh... i don’t want to get fired.”

  He winked, and Gaster wanted to smack him across the face and kiss him deeply all at once.

  Gaster went to sign something biting, but was cut off when Sans slipped in him with ease, with a heavy, panting grunt as Gaster scrabbled at the table. He pulled out, hesitating, and Gaster composed himself enough to ask his questions.

  ‘Something... The matter?’

  “naw, it’s just...”

  Sans shifted, his cock throbbing, and Gaster hissed at the sensation.

  “y-ya feel really, uh, r-really different, heh. n-never, never done something like this with, uh--”

  ‘Someone like me?’

  “yeah.”

  Sans rolled his hips, shrugging, going with the flow at this point, and Gaster opened his mouth to cry out, hands squeezing at Sans’ shoulders. He gripped hard enough to hurt, groping at Sans’ ribs in a desperate bid to give him stimulation. Sans tugged at Gaster in uneven, stuttering strokes, his composure not holding under the strain, under the stimulation. He was panting, teeth parted.

  They shouldn’t be doing this, it was the Royal Scientist’s office during work hours, it could be seen from the ground level under certain conditions, they could be blackmailed. It was a thrilling, dangerous thought, scandalous.

  “geeze, gaster...”

  There was his name again, and it spurred Gaster on, made him open himself that little bit wider, Sans gripping at his hips.

  “y-you’re... really tight.”

  One palm braced to the mahogany desk, one to the hip, and Sans fucked him, shallowly, lightly at first, before his instinct compelled him to finally push harder, to press deeper. His pants were bunched around his ankles and his belt jingled with every shuffle, every stuttered, gasping breath. Gaster couldn’t moan, couldn’t cry out, couldn’t plead for him to go faster, harder, and so clattered his hand against the desk to voice his pleasure, squeezing Sans internally.

  Sans, to his own shock, yelped. And Gaster wanted to make him do it again. He could feel his cock pulse against his insides, and Gaster was close himself.

  There was no noise outside of their fucking, and Sans’ increasingly shallow breaths that were moving into gasps, until he couldn’t stand to take in even a lungful of air.

  “oh god, oh...”

  Sans’ face split into a wicked grin, high on sex and mirth, always ready to push buttons.

  “ _daddy._ ”

  Gaster cringed, Sans’ low laughter doing nothing to soften his response, with such force that he had to stop himself from shoving Sans away.

  ‘Don’t make this weird!’

  “it’s already--” Sans drew his hips back for a long, deep thrust, before slipping back in with a grunt, “-- pretty weird, doc.”

  Sans scrunched his eyes shut, hissing through his teeth, finally falling into a pattern as he jerked Gaster off, a consistent one that was going to make him cum harder than he ever had, they shouldn’t be doing this, they shouldn’t be doing this.

  “d-doc?”

  Gaster opened his eyes, just enough to make out Sans’ huffing, tinted face.

  “there’s a cute dog eraser, the little display one? that was on your desk? i want it.”

  Gaster’s eyes shot open, bringing his hands up to sign. ‘Oh come on, you know I very rarely buy myself those sorts of things!’

  Sans thrusted languidly before coming to a complete stop. He shuddered.

  ‘Fine!’

  “i want a raise.”

  ‘You can’t possibly be--”

  Sans squeezed firmly at Gaster’s cock.

  ‘Fine! Fine! God almighty, just move!’

  “rad. first dibs in the staff canteen?”

  ‘Blackmail! This is blackmail, Sans!’

  Gaster felt Sans’ cock slip out, and this couldn’t happen, not when he was so close, not when he was about to cum.

  ‘ _Fine!_ ’

  And Sans flashed that stupid grin, the one that never left his face, but backed with his own amazing power to irritate in an endearing way, before cramming himself back in to the hilt, fucking as fast as his own body would allow him, deeper and deeper until Gaster’s body was squeaking against the table. Building, building, building, building--

  Gaster arched his back, cumming into Sans’ palm as he jerked, as the feeling tore through him, Sans was jerking him off, _Sans was jerking him off_ , and this day would be enough to fuel his fantasies for the next one thousand years. He rode it out, eyes clamped shut, bracing himself against the desk, his jaw open. He felt cum splatter against his front, until he went limp, out of it, blissful.

  “huh,” Sans panted, “neat.”

  With a long, throaty groan Sans came, his thrusts dying down to become shallow, all of his bones tensing as the feeling coursed up and down his body, leaving him with a goofy expression that Gaster would only describe as adorable. Gaster lay there, panting, Sans hunched over him. He moved to cup Sans’ face. Sans had his eyes shut, not noticing, and he pulled out. Gaster let his hand flop. He rapped his knuckles on the table, grinning, high on afterglow.

  ‘The raise... Will require careful manoeuvring to justify--’

  Confusion flitted across Sans’ features before it clicked. “doc. gaster, holy shit, i was jokin’. one of those ‘getting swept up in it’ kinda moments, i don’t actually want the shit you promised. what, you were serious?”

  ‘No,’ he lied. ‘I was playing along. Tone gets lost in translation when you can’t speak.’  
  
  Sans nodded. “ah.”

  They were silent, both unsure of how to continue, the sexual tension having passed to give way to something else. Gaster didn’t want it to warp into genuine tension, and that gnawing feeling settled in him again. Mistake, this was a mistake. One he wanted to repeat over and over.

  “so, uh... what... i...”

  Sans shrugged, wheezing.

  “the fuck do we do now?”

  Gaster could not answer. He had many options, and one stuck prominently in his mind, but he could not voice it, could not force his feelings onto Sans.

  “hey, hey... i got an idea.”

  Gaster blinked, sitting up on the table, feeling the cum slip out of him. He would need to clean that up.

  “so, say it’s your average workday, and we’re both pretty much done...”

  Sans clapped, as if he were having a eureka moment.

  “... whaddya say we make this a thing?”

  Gaster was going to squeal.

  “we come up here, fuck like rabbits, then go on about our days? only if you’re down. if you wanna write this off, we write it off.”

  Oh. That sort of thing.

  Gaster was going to say no.

  ‘That is an attractive prospect.’

  Oh God no, he wasn’t doing this. He wasn’t. He wanted more than what Sans was offering, he couldn’t agree to this.

  ‘We are both consenting adults, and as long as this doesn’t get out--’

  “it won't.”

  ‘... And as I said before, if you change your mind, you do not need to worry about job security. If I fire you, it will be because you have made a catastrophic, possibly fatal, mistake.’

  “thanks, doc, for puttin’ my mind at ease,” he drawled.

  Say no! Rescind the agreement!

  ‘And the idea of you sweating and gasping again isn’t an unpleasant one,’ he leered, mentally kicking himself, yet unable to stop his motions.

  Stupid man, stupid, stupid! Stop this! Stop!

  “heh. likewise.”

  Ooh, his teeth were sort of pointy. That was cute-- _No!_

  ‘Would you mind helping me sort my desk? I believe I have a meeting.’

  Sans grabbed his ass, squeezing, grinning lasciviously, primally. In a way that showed he only wanted the sex, and the companionship.

  Gaster forced a smile.

 

* * *

 

  “well doc, i guess i’m all done for the day...”

  ‘It is ten-thirty, Sans. It has been an hour.’

  “i know, right? i’m such a hard worker. maybe i should get a promotion, or a... hmm, maybe a reward, or somethin’...”

  They were in Gaster’s office again, with the added luxury of privacy, given that Gaster had locked the door when he would come in. It was a good habit to develop. It had been two weeks, and they had fucked only a few more times, their schedules not accounting for their new hobby.

  ‘My God, just come right out with it.’

  “i mean, ya can’t let such good work go unappreciated...”

  ‘You’re actually doing this. I want to sleep with you, but the acting is unnecessary. I am saying yes.’

  “because, gosh, i’ve just been workin’ so hard...”

  Gaster let out a weary, weary sigh, before gathering up the papers on his desk into a neat pile and slipping them under it. He unplugged the desk fan, his new one, before setting it on the ground as well.

  Gaster’s expression was one of pain.

  ‘I suppose if you have done your duties, then it is only fair...’

  He cringed.

  ‘Sans, Sans I can’t do this. This... I’m not an actor, the table is clear, we can--’

  Sans looked on, hopeful, the wicked gleam in his eyes apparent.

  ‘... You deserve a reward, for your diligence, even though it is not even the afternoon, even though I am certain you have been napping at your desk, even though you... You...’

  Sans was thumbing at his own erection, the tip of which he had exposed. Light glinted off the pre-cum.

  ‘U-up on the table, now.’

  “i can lie back and do nothin’?”

  ‘Yes. Although pro-activeness would be greatly appreciated.’

  “well, i don’t wanna disappoint, do I?”

  Sans’ phone buzzed, and his eyes went wide. He slipped it out of his pocket.

  “h-hold on one sec, i gotta take this.”

  ‘Of course.’

  Ah, it was a text from Papyrus. ‘JUST TO LET YOU KNOW I’M AT MY COOL NEW FRIEND’S HOUSE, THE BLUE LADY! I THINK YOU’VE MET? ANYWAY, I LEFT YOU A MEAL IN THE FRIDGE! HAVE FUN AT WORK!’

  Sans smiled, earnestly, and Gaster couldn’t help but smile along. He slipped the phone in his back pocket, too high on lust to think to lock the screen, eager to get started. He felt cool, slick tendrils wrap around his ankles, slowly working their way upwards, making him shake with anticipation.

  “that’s a hell of a party trick.”

  ‘Oh, I try’.

 

  
  Oh, Sans was calling! How lovely, Papyrus hadn’t expected a call. He was reclining on Undyne’s couch, and she was busy pottering in the kitchen. What a surprise this was.

  “WHY HELLO SANS, HOW--”

  Papyrus heard panting, heavy, heavy panting. He heard rustling, like that of grass, and muffled voices. Sans whined.

  He was...

  He was...

  Running from something! God, Sans, no, he was too unfit to run, his frame was too weak, and lord knows he couldn’t survive a fight! Poor, soft Sans, what kind of trouble had he gotten himself into? Papyrus listened closely, hearing Sans’ voice ring out through the tinny speakers.

  “p-please, p-please, gaster...”

  Oh God! He was being kidnapped for devilish scientific experiments! That Gaster must have been a bad egg, no wonder he was so scary looking!

  “fuckin’.. god...”

  Such language! This may be it, this may be the end of the line, the end of Sans the skeleton, who has done his very best. Papyrus was about to burst into tears, he needed to leave to go save his brother, needed to--

  “more, more, _fuck!_ ”

  ... Wait, what?

  Papyrus let out a loud screech of panic, of all encompassing embarrassment as he realized what he was hearing.

  “SANS? SANS! SANS, _GOOD GOD!_ ”

  The noises stopped, and Papyrus heard the clattering of a phone speaker, heard the rush of wind and Sans jam the receiver to his face.

  “h-hey papyrus, what’s up?”

  “SANS, WHAT WAS THAT?”

  Sans would have vomited if he could out of horror, as he slapped Gaster’s tentacles away. Gaster took the hint, mortification slowly setting in.

  “joggin’, i was joggin’, tell him, gaster!”

  Gaster looked at him blankly.

  ‘Yes, I’m sure he will be able to hear the signing. Brilliant, Sans. Truly exceptional.’

  “ _SANS! WE BOTH KNOW YOU WEREN’T JOGGING!_ ”

  Sans was going to toss himself into the lava. He had never been more embarrassed in his life. Never.

  Gaster gave him a pat on the shoulder, desperate to know if their secret was safe.

  ‘How good is your brother at lying? Or keeping secrets?'

  “u-uh, you see, the thing is--”

  Papyrus’ voice blared, echoing through the room.

  “ **UNDYNE! UNDYNE, I JUST HEARD SANS BONE-DEEP IN THE ROYAL SCIENTIST AND I NEED TO BE CONSOLED IMMEDIATELY!** ”

  Gaster turned to Sans, speaking some of the few words Sans would ever hear from him, and Sans would have been impressed if not for his own crushing embarrassment.

  “ _Uh-oh_.”

  Gaster was far, far too old for this.

**Author's Note:**

> the husk oneshot is coming, i just have a few fics between then and now, if you were wondering


End file.
